The Syrian army and rebels on Wednesday sent reinforcements to Aleppo to join the intensifying battle for the country's second city, as UN chief Ban Ki-moon urged the world "to stop the slaughter."
UN peacekeeping chief Herve Ladsous said he had told Syrian officials that without a significant reduction in violence, the remaining 150 observers would leave on the expiry of the "final" 30-day extension of the mission's mandate agreed by the Security Council on July 20.
Russia, meanwhile, ramped up its criticism of Western policy as helicopter gunships strafed several neighbourhoods of the commercial capital, causing deaths and injuries, the Syrian Observatory for Human Rights said.
White House spokesman Jay Carney condemned the use of attack helicopters in the conflict as "another indication of the depth of depravity" of President Bashar al-Assad's regime.
Aiming to regain the diplomatic initiative, Russia's UN ambassador Vitaly Churkin said Moscow was ready to host talks between the Syrian opposition and the regime.
"We are ready to give the opposition and the government a platform in Moscow to forge contacts to unify the opposition and for negotiations with the government," he said.
On the ground, clashes raged in Aleppo's central Al-Jamaliya neighbourhood, near the local headquarters of the ruling Baath party. In Kalasseh in the south of the city, rebels set a police station ablaze, the Observatory said.
A rebel spokesman told AFP via Skype that a "large number" of troops have been moved from the northwestern province of Idlib to Aleppo.
Colonel Abdel Jabbar al-Oqaidi of the rebel Free Syrian Army said he believed the reinforcements were being sent because of the intensity of clashes in Aleppo, where several districts were "liberated" on Monday.
A Syrian newspaper journalist confirmed the rebels were also reinforcing.
"Hundreds of rebels from all over the north of Syria are arriving in Aleppo, which appears to have become the decisive battle," the journalist told AFP.
The Britain-based Observatory also reported clashes in the Al-Hajar Al-Aswad district of Damascus, one of the last remaining rebel bastions after 10 days of fighting in the capital.
Helicopter gunships and heavy machinegun fire pounded the embattled southern neighbourhood, the Observatory said.
Nationwide, the monitoring group put the death toll at 108 by Wednesday evening -- 57 civilians, 36 soldiers and 15 rebels, while it said 158 people were killed across Syria on Tuesday.
In Hama province in central Syria, a couple and their two children were killed as they tried to flee shelling. A video distributed by the Observatory showed grisly footage of the bodies.
UN chief Ban, in Bosnia which suffered a genocide in Srebrenica in 1995, told parliament: "I make a plea to the world: 'Do not delay... Act now to stop the slaughter in Syria.'"
"Today the international community is being tested in Syria," Ban said. "The echoes are deafening: An accelerated slide to civil war. Growing sectarian strife. Villages and children butchered."
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As the violence raged, the UN Supervision Mission in Syria, deployed to supervise an April truce that never took hold, sent home half of its 300 unarmed military observers.
-- 'Reduced format' --
"UNSMIS (is) in a reduced format," said Ladsous, who arrived in Damascus on Tuesday.
"About half the military observers have been for the time sent back to their countries, so the mission operates on a reduced basis, reduced in numbers, reduced in team size in the provinces, and does what it can," he said.
Ladsous said he had stressed to Syrian officials that any extension of the observer mission beyond the 30 days set by the Security Council would require "very specific and sustainable progress on the level of violence, which should subside substantially, and on the use of heavy weapons."
Western governments have expressed scepticism about the chances that violence will subside sufficiently during that time, but Russia, increasingly critical of Western policy, has argued that the observers should stay.
Russian Foreign Minister Sergei Lavrov lashed out at the United States for backing the armed opposition, saying a US failure to condemn the July 18 bombing that killed four top Syrian security officials meant it was justifying terror.
And a Russian foreign ministry statement said a new round of EU sanctions agreed this week, which allows for the inspection of vessels and planes suspected of carrying arms to Syria, amounted to an air and sea "blockade."
It said experts needed to look into the EU legislation to see whether it was in line with international law.
Rights group Amnesty International, meanwhile, warned on Wednesday about disturbing reports of "summary executions" by both Syrian troops and rebels, calling them "serious violations of international law."
Turkey indefinitely closed three border crossings to Turkish nationals trying to get into Syria, citing security concerns.
The UN High Commissioner for Refugees in Geneva said about 300 people fled from Syria into Turkey on Tuesday night.
Two Syrian brigadier generals also entered Turkey on Tuesday, bringing to 27 the number of generals who have defected, a foreign ministry official told AFP.
Syria also confirmed that the head of its mission in Cyprus, Lamia al-Hariri, has defected.
Meanwhile, Saudi Arabia is proposing a UN General Assembly resolution which will highlight a Syrian government threat to use chemical weapons, its UN envoy said.
Ambassador Abdallah al-Mouallimi told reporters the resolution would be submitted in coming days and he hoped for a vote "probably early next week."